1. Field of the Invention
This invention relates to camera shutters, particularly to shutters in which electromagnets control release of front and rear shutter curtains.
2. Description of the Prior Art
A known electromagnetically controlled shutter arrangement supplies power to an electromagnet or electromagnets beforehand to attract and retain an armature, and the thus supplied current is shut off as required to separate the armature from the magnet. Thus, the shutter is controlled by utilizing this power. While this type of shutter has the advantage of consuming only a small amount of power, it is not necessarily advantageous with respect to manufacturing costs. Abrasion of an attracting surface occurs and an equalizer for causing the armature uniformly to adhere to the electromagnet as well as a charging mechanism for pressing the armature against an attracting surface are necessary.
In contrast, an electromagnetically controlled shutter of the so-called attracting type releases shutter curtain checking devices by supplying power to an electromagnet for attracting an armature. This introduces some shortcomings with respect to power consumption, but reduces manufacturing costs.
Unfortunately, shutters of the latter type have the drawback of permitting the shutter speed to become gradually slower when the power source battery has been used for a long time and is unable to provide sufficient power. This is so because power is supplied to the rear curtain controlling electromagnetic after the power supplied to the front curtain electromagnet has caused a voltage drop. Thus, deterioration of the battery affects the electromagnet for the rear curtain more than the electromagnet for the front curtain. This weakens the attractive power of the rear curtain electromagnet and results in increasing time lags from the moment power is supplied to the rear curtain electromagnet until the moment the rear curtain is released as the battery deteriorates. To eliminate this shortcoming, power may be stored in a capacitor and voltage may be boosted, as required, and such stored power may be used. However, such methods require additional circuitry which demand additional space, and are thus not advantageous. Also, a constant current circuit may be used. However, the disadvantage of such a circuit is the need for an additional battery to compensate for voltage losses.